To produce a more efficient vehicle, hybrid vehicle powertrains combine an electric motor(s) and a conventional internal combustion engine. Torque from the engine and the electric motor(s) is typically channeled to the vehicle's driven wheels via a transmission. Efficiency of a hybrid vehicle powertrain is typically related to the driving conditions and percentage of time that the engine must be run in addition to or in place of the electric motor to power the vehicle.
Some hybrid powertrains employ a single electric motor in combination with the engine. In such powertrains, transmission output, and hence vehicle speed, is directly related to the speeds and torques of the engine and the electric motor. Other hybrid powertrains employ two electric motors in combination with the engine to power the vehicle. Additionally, a vehicle may employ purely electric propulsion. In such a case, the vehicle's powertrain will have one or more motor-generators and no internal combustion engine.
In either a hybrid or purely electric powertrain the electric motor(s) are operatively connected to a transmission having planetary gearing such that torque and speed of the electric motors may be selected independently of vehicle speed and desired acceleration. In a hybrid powertrain, control of the engine is typically achieved by varying individual torque contribution from the electric motor(s). Thus, such hybrid and purely electric powertrains may each provide selectable torque contribution from their electric motors, and in the case of the hybrid powertrain may similarly provide a selectable torque contribution from the engine to drive the subject vehicle.